This invention relates to an imaging system using Focal Plane Arrays (FPAs), and in particular but not exclusively to such imagers, imaging in the Infra Red (IR) wavebands.
IR imaging systems are becoming more important in many fields now, in particular military, security and search and rescue applications. Early IR imagers employed a small number of detector elements, across which was scanned an IR image of the scene via a system of mirrors and polygons. More recent developments include imagers based on 2 dimensional arrays of detector elements, so called staring arrays, which require no scanning to produce a useful image of the scene. The dwell time available for each detector element in such systems is considerably increased over earlier scanner systems resulting in significantly improved system performance being achievable from comparable detector materials. The IR system designer can choose whether to exploit this increase in performance or use a lower performance detector material to achieve a similar sensitivity as in the earlier scanner systems. High system performance is typified by imagers based on arrays of Cadmium Mercury Telluride cooled to liquid nitrogen temperatures, whilst conventional levels of performance are achieved by imagers based on Schottky barrier arrays and pyroelectric ceramics These latter systems offer significant advantages in terms of cost and or logistical support requirements (such as coolant supplies) over the high performance systems.
Unfortunately, several disadvantages of FPA imagers must be overcome to provide performance comparable with conventionally scanned imagers. Current FPAs are only available in limited pixel counts, typically 128.times.128 or 256.times.256 elements, which is insufficient to match the spatial resolution of the best scanned imager systems. Eventually, the development of suitable fabrication technologies will overcome this problem, resulting in large pixel densities.
A more fundamental problem, however, concerns the basic physics of imaging via a focal plane array. This is shown schematically in FIG. 1a, where a single row of elements from the detector array is considered. For simplicity the elements are considered square in shape, of length given by A, and are fabricated with a pitch P. The modulation transfer function (MTF) of a single element in the array is given by the modulus of the sinc function, as shown in the figure, with the first zero occurring at a spatial frequency of 1/A. Since an array of such elements is used, this MTF as shown in FIG. 1a is convolved with a series of delta-functions separated by spatial frequencies of Fs(=1/P), the sampling frequency. As shown in the figure, this results in a folding of the MTF curve into the area between 0 and Fs/2. Spatial frequencies higher than Fs/2 which are present in the image are reproduced by the array as lower, alias frequencies in the range 0 to Fs/2. For 2 dimensional arrays the effect is much worse than FIG. 1A shows, since aliasing occurs in both axes simultaneously. The effect is similar to conventional data sampling limitations, governed by Nyquist's Theorem, except that it occurs in the spatial domain rather than in the post detection electronics.
For staring systems, therefore, the MTF is limited by twice the detector pitch, and the full MTF available from the detector geometry cannot be exploited as it would in a scanning system. One technique which has been widely used to overcome this limitation is microscan, or mechanical interlace. In this technique, the image of the scene is moved across the detector array, when the device is not imaging, by a fraction of the inter element pitch, such that an integral number of steps fit into the pitch. The display of the subsequent field of data is shifted by a corresponding amount to ensure the fidelity of the reconstructed image. The effect of microscan is shown in FIG. 1b, which considers a first order microscan, in which the image is moved by 1/2P. The MTF of the individual elements remain unchanged, however the sampling frequency is multiplied by the microscan factor, in this case x2. As a result a much larger portion of the MTF can be utilised before aliasing is encountered.
Microscan therefore permits the MTF of scanning systems to be achieved in staring systems. Different orders of microscan are frequently adopted, such as 2.times.2 or 3.times.3, the numbers representing the number of steps in each axis per pixel pitch. The optimum choice of microscan order depends on the relative size of the element and sampling pitch and the effect of other factors such as the optical MTF.
FIG. 2 shows a typical imager 1 for implementation of microscan. The imager 1 comprises a lens 2 focusing radiation on to an imaging array of thermal detectors 3. Such detectors require the radiation incident on them to be modulated by a chopper 4, driven by motor 5 energised by battery 6, in order for the detection process to work. A leading edge of the chopper 4 scans across the array 3 synchronously with readout from the array by the electronic circuit 7. Radiation from the scene is incident on the detector via a mirror 8. Microscan is achieved by tilting the mirror 8 while the entire detector is covered by the chopper 4. This implementation has the following disadvantages:-
1. The microscan mechanism can be quite complex, since the mirror is ideally required to tilt in two orthogonal axes at a relatively high speed;
2. For maximum efficiency, the detector would be operated in consecutive fields, with the gaps between the chopper blades being exactly the same size as the detector itself. This would result in the entire detector being closed to radiation only for an instant. For the microscan mirror to move without degrading the image quality, this must be extended to a finite period--a blanking period--which can be a significant portion of the active field period. As a result the efficiency of the system is reduced;
3. The requirement of a fold in the optical path, for the reflective microscan to operate, limits the lowest f-number which the lens can achieve without vignetting, typically to greater than f/2.8;
4. The backworking distance of the lens must be maximised to fit the optical fold in; and
5. The entire assembly is difficult and costly to manufacture, requiring precision design and fabrication due to the large number of components competing for space close to the focal plane.
It is an object of the present invention to provide an imager which overcomes at least some of the above problems.